Facilitating Uniform Large-Scale MoS2, WS2 Monolayers, and Their Heterostructures through van der Waals Epitaxy

The fabrication process for the uniform large-scale MoS2, WS2 transition-metal dichalcogenides (TMDCs) monolayers, and their heterostructures has been developed by van der Waals epitaxy (VdWE) through the reaction of MoCl5 or WCl6 precursors and the reactive gas H2S to form MoS2 or WS2 monolayers, respectively. The heterostructures of MoS2/WS2 or WS2/MoS2 can be easily achieved by changing the precursor from WCl6 to MoCl5 once the WS2 monolayer has been fabricated or switching the precursor from MoCl5 to WCl6 after the MoS2 monolayer has been deposited on the substrate. These VdWE-grown MoS2, WS2 monolayers, and their heterostructures have been successfully deposited on Si wafers with 300 nm SiO2 coating (300 nm SiO2/Si), quartz glass, fused silica, and sapphire substrates using the protocol that we have developed. We have characterized these TMDCs materials with a range of tools/techniques including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), micro-Raman analysis, photoluminescence (PL), atomic force microscopy (AFM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and selected-area electron diffraction (SAED). The band alignment and large-scale uniformity of MoS2/WS2 heterostructures have also been evaluated with PL spectroscopy. This process and resulting large-scale MoS2, WS2 monolayers, and their heterostructures have demonstrated promising solutions for the applications in next-generation nanoelectronics, nanophotonics, and quantum technology.


INTRODUCTION
Transition-metal dichalcogenides (TMDCs) such as MoS 2 , MoSe 2 , WS 2 , and WSe 2 are two-dimensional (2D) van der Waals (VdW) layered materials. Unlike graphene, TMDCs are semiconductors that could offer, in particular, bandgap engineering properties through both their chemical compositions and their number of layers. 1,2 The applications for using TMDCs are very promising in the area of transistors, 1 lightemitting diodes, 3,4 photodetectors, 5 sensing 6,7 and memory devices, 8 as well as the potential substitution for Si in conventional electronics 9 and of organic semiconductors in wearable and flexible systems. 10 The current fabrication processes for these emerging TMDCs include exfoliation, 1,11 hydrothermal process, 12 physical vapor deposition, 13 transition-metal oxide sulfurization, 14 electrochemical deposition, 15 thermolysis of transitionmetal chalcogenide compounds 16,17 and chemical vapor deposition (CVD). 18−20 The majority of TMDCs fabricated by these techniques are in the form of flakes with the sizes in the range of a few hundred square micrometers in area. However, the challenge for large-scale fabrication of TMDCs is to provide a reliable complementary metal-oxide-semiconduc-tor (CMOS)-compatible process for the integration of 2D TMDCs on a desired wafer-scale substrate. 2,21 We have been working on the synthesis of chalcogenide materials using vapor phase deposition processes 22−27 such as CVD, atomic layer deposition (ALD), and van der Waals epitaxy (VdWE). Apart from offering conformal coating and stoichiometric control of thin film compositions, these processes are scalable and compatible with a range of substrates. In particular, VdWE has been demonstrated to perform the epitaxy of layered TMDCs on the substrates even with lattice constants mismatch. 28−30 In this paper, we have developed the fabrication process for the uniform large-scale MoS 2 , WS 2 TMDCs monolayers and their heterostructures by VdWE through the reaction of MoCl 5 or WCl 6 precursors and the reactive gas H 2 S to form MoS 2 or WS 2 monolayers, respectively. The heterostructures can easily be achieved by changing the precursor from WCl 6 to MoCl 5 once the initial WS 2 monolayer is fabricated or switching the precursor from MoCl 5 to WCl 6 after MoS 2 monolayer has been deposited on the substrate.

EXPERIMENTAL SETUP
The VdWE apparatus we developed is shown schematically in Figure  1. The precursors�MoCl 5 (99.6% pure from Alfa Aesar) and WCl 6 (99.9% pure from Sigma-Aldrich)�were kept in bubblers inside the dry N 2 purged glovebox. The MoCl 5 /WCl 6 vapors were delivered by high-purity argon gases through the mass flow controllers (MFCs) to the VdWE apparatus with the flow rate of 300 standard cubic centimeters per minute (sccm). The system equipped with a bespoke furnace with three heating zones, individually controlled by proportional integral derivative (PID) controllers, with maximum temperature of 1200°C and temperature uniformity of ±3°C can be achieved over a length of 450 mm to facilitate the uniform large-scale deposition of TMDC monolayers. The reactive gases were H 2 S mixed with another argon gas through individual MFCs with the flow rates of 50 and 300 sccm, respectively. All the gases were purified by passing through the individual point of use purifiers (SAES MicroTorr) and the moisture level of all gases were monitored by the dewpoint sensors (Michell Instrument Pura pure gas trace moisture transmitters) before entering the VdWE reactor. The typical moisture readings of the Ar and H 2 S/Ar mixture were −99.6°C dp (∼7 ppb) and −90.2°C dp (∼42 ppb), respectively. The process was set at 30 mbar using a pump (Vacuubrand MV 10C NT Vario) with a pressure controller for the entire deposition. With this VdWE apparatus, uniform large-scale TMDC monolayers have been successfully deposited on various substrates, including 300 nm SiO 2 /Si, quartz glass, fused silica, or c-plane sapphire. The sizes of the substrates were typically 25 mm × 25 mm, however up to a 40  mm × 100 mm substrate can be loaded into the VdWE apparatus, which consists of a 50 mm O.D. × 1000 mm long quartz reaction tube. The substrates were cleaned with acetone using an ultrasonic bath at 50°C for 10 min, then rinsed with isopropanol and deionized water and subsequently subjected to blow drying with N 2 gas. The temperatures for the growth of MoS 2 and WS 2 monolayers were set at 850 and 900°C, respectively. The reactive H 2 S gas and MoCl 5 /WCl 6 precursors were introduced to the VdWE system once the furnace reached the set temperature. MoS 2 /WS 2 were formed after the MoCl 5 /WCl 6 precursors met with H 2 S gas after the injection tube inside the quartz reaction tube. With sufficient amount of MoCl 5 / WCl 6 precursors flux, MoS 2 /WS 2 monolayers can be uniformly deposited on the substrates and the resulting MoS 2 /WS 2 monolayers have a tendency to be polycrystalline, because of the high flux of precursors. Although the substrates might affect the deposition of TMDCs, we did not see significant differences in the quality of the MoS 2 , WS 2 monolayers, and their heterostructures on the substrates we used. This is probably due to the VdWE process can overcome the mismatch of substrate lattice constants. To achieve uniform MoS 2 and WS 2 monolayers, a deposition time of 4 and 5 min was required for the MoS 2 and WS 2 monolayers, respectively.

RESULTS AND DISCUSSION
We have achieved large area MoS 2 and WS 2 monolayers as shown in Figure 2a on quartz glass and in Figure 2b on 300 nm SiO 2 /Si substrates, respectively. These results have demonstrated that wafer scale deposition of MoS 2 and WS 2 monolayers is feasible through a modification of the VdWE system with a larger reaction chamber.
Raman spectroscopy was performed for the initial study of the quality of the VdWE-grown MoS 2 and WS 2 monolayers on quartz glass and 300 nm SiO 2 /Si substrates, using a Renishaw Ramascope. MoS 2 monolayer and WS 2 monolayer samples were excited using 532 and 473 nm excitation lasers, and the Raman shift spectra for MoS 2 and WS 2 are shown in Figures 2c and 2d, respectively. As shown in Figure 2c, two MoS 2 Raman peaks, E 2g 1 in-plane phonon mode and A 1g out-of-plane phonon mode were revealed at 384.0 and 403.7 cm −1 , respectively. The number of MoS 2 layers can be evaluated by the energy difference between these two Raman peaks (Δ). 31 From Figure  2c, the Δ value is 19.7 cm −1 for the VdWE-grown MoS 2 monolayer, which is similar to the reported literature. 31 On the other hand, in order to reduce the second-order 2LA phonon mode in the WS 2 Raman measurement, 32 a 473 nm laser was used to reveal two WS 2 Raman peaks, E 2g 1 and A 1g at 359.2 and 419.4 cm −1 , respectively. Again, the Δ value can be also used to evaluate the number of WS 2 layers. 32 From Figure 2d, the Δ value is 60.2 cm −1 for the VdWE-grown WS 2 monolayer, which also matches with the literature. 32 The photoluminescence (PL) spectroscopy from the VdWEgrown MoS 2 monolayer on quartz glass and WS 2 monolayer on 300 nm SiO 2 /Si substrates were studied using the same Raman microscope. Two excitonic peaks A and B, at 666.3 nm (1.86 eV) and 614.3 nm (2.02 eV), respectively, were found in the PL spectrum of VdWE-grown MoS 2 monolayer on a quartz glass substrate, as shown in Figure 2e. These results are similar to the reported literature. 33 On the other hand, the PL spectrum of VdWE-grown WS 2 monolayer on 300 nm SiO 2 /Si substrate confirmed the direct band emission at 616.1 nm (2.01 eV), as shown in Figure 2f. Again, this result agrees with the literature reports. 34 Furthermore, the PL spectra mapping was performed to study the uniformity of large-scale VdWE-grown WS 2 monolayer on a 300 nm SiO 2 /Si substrate. The map of the PL emission at 2.01 eV shown in Figure 3 reveals very good uniformity of the WS 2 monolayer over an area of 35 mm × 50 mm. This has been achieved by our in-house-built apparatus, and this process could be scalable for even large wafer-scale processes if a larger reactor is available.
X-ray photoelectron spectroscopy (XPS) was performed to study the compositions of these VdWE-grown MoS 2 and WS 2 monolayers using a Thermo Scientific Theta Probe XPS System. For the MoS 2 monolayer, two core levels, Mo 3d and S 2p, have been investigated. As shown in Figure 4a, two MoS 2 peaks, Mo(IV) 3d 3/2 and Mo(IV) 3d 5/2 , were found at 233.0 and 229.9 eV, respectively. In the same spectrum, S 2s peak was observed at 227.2 eV and a peak at 236.0 eV was assigned to Mo(VI) 3d 3/2 , indicating a small amount of oxidation, which resulted from the sample being exposed to the ambient environment. Note that a Mo(VI) 3d 5/2 peak overlaps with Mo(IV) 3d 3/2 at 233.0 eV. For the MoS 2 S 2p core level, two peaks labeled in Figure 4b as S 2p 1/2 and S 2p 3/2 corresponding to MoS 2 were found at 163.9 and 162.7 eV, respectively. In addition, using a semiquantitative method to investigate the ratio of elements, the atomic ratio of S/Mo was determined to be ∼1.93 with a slight S deficiency. These results are consistent with the literature. 35 On the other hand, for the WS 2 monolayer, two core levels have been studied: W 4f and S 2p. As shown in Figure 4c, two WS 2 peaks, W(IV) 4f 5/2 and W(IV) 4f 7/2 , were found at 35.2 and 33.0 eV, respectively, and in the same spectrum, two peaks at 38.5 and 36.3 eV were assigned to W(VI) 4f 5/2 and W(VI) 4f 7/2 , which again indicate a small amount of oxidation. Also, note that the W(VI) 4f 5/2 peak overlaps with W(VI) 5p 3/2 at 38.5 eV. For the WS 2 S 2p core level, two peaks labeled in Figure 4d as S 2p 1/2 and S 2p 3/2 , corresponding to WS 2 , were found at 164.0 and 162.8 eV, respectively. In addition, the atomic ratio of S/W was found to be ∼1.96, with a slight S deficiency. These results also agree very well with the literature. 36 In order to evaluate the crystalline structures of these VdWE-grown MoS 2 or WS 2 monolayers, commercially available 40 nm SiO 2 membranes TEM grids with 200-nmthick Si 3 N 4 supporting frames (PELCO Silicon Dioxide Support Films for TEM) were used to directly deposit these MoS 2 and WS 2 monolayers on this type of TEM grid. The optical image of as-deposited MoS 2 monolayer on TEM grid is shown in Figure 5a with a 532 nm laser spot on the center of    Figure 5b. Again, two characteristic MoS 2 Raman peaks, E 2g 1 and A 1g modes were found at 385.8 and 402.9 cm −1 , respectively, with a Δ value of 17.1 cm −1 for the VdWE-grown MoS 2 monolayer on a 40 nm SiO 2 membrane TEM grid. Note that the Δ value appears to be less than that typically reported for the MoS 2 monolayer, because of the weak Raman signal from the sample, which increased the experimental uncertainty. In addition, the smaller Δ value could be also due to softening of the A 1g mode. The E 2g 1 mode is insensitive to substrates but the A 1g mode is sensitive to charge density. 37 Despite these issues, however, the monolayer nature has been revealed. In the PL spectrum, shown in Figure 5c, only the A excitonic peak at 661.1 nm (∼1.88 eV) was found for this sample on a 40 nm SiO 2 TEM membrane, whereas the B exciton could be only weakly detected. The sample was inspected using scanning tunnelling electron microscopy with a high-angle-annular-dark-field (HAADF-STEM), using a FEI Talos F200x system (USA), operating at 200 kV and equipped with an energy-dispersive Xray spectrometer (EDX) system. The TEM image shown in Figure 5d has revealed the polycrystalline nature of the VdWEgrown MoS 2 monolayer on a 40 nm SiO 2 TEM membrane, and the grain sizes are ∼10 nm. The selected-area electron   Figures 5f and 5g, the Mo and S, respectively, were quite uniform over the measured area.
The optical image of as-deposited WS 2 monolayer on TEM grid is shown in Figure 6a with a 532 nm laser spot on the center of the WS 2 /40 nm SiO 2 membrane. The Raman spectrum of the WS 2 monolayer/40 nm SiO 2 sample is shown in Figure 6b. Two WS 2 Raman peaks�2LA phonon mode and A 1g mode�were found at 352.9 and 416.0 cm −1 , respectively. In addition, as shown in Figure 6c, the direct band emission at 620.0 nm (2.00 eV) was revealed from the PL spectrum. Again, these results agree with the literature reports. 26,34 The TEM image shown in Figure 6d has revealed the polycrystalline nature of VdWE-grown WS 2 monolayer on 40 nm SiO 2 TEM membrane, and the grain sizes are ∼10 nm. The SAED pattern shown in Figure 6e also confirmed the polycrystalline structures of this WS 2 monolayer. The elemental mapping was performed in the STEM-EDX mode. As shown in Figures  6f and 6g, the W and S atoms, respectively, were quite uniform over the measured area.
A MoS 2 /WS 2 monolayer heterostructure on the fused silica substrate was prepared for further investigation with the abovementioned VdWE process. WS 2 monolayer was first grown on a 25 mm × 25 mm fused silica substrate, followed by the second MoS 2 monolayer grown on the top of a WS 2 monolayer/fused silica sample. As the Raman spectrum shown in Figure 7a, two typical MoS 2 E 2g 1 and A 1g peaks are revealed, along with the WS 2 peaks labeled as WS 2 (2LA-2E 2g 2 ), WS 2 (2LA-E 2g 2 ), WS 2 (2LA+E 2g 2 ), and WS 2 (A 1g ). The band alignment of MoS 2 /WS 2 monolayer heterostructures has also been evaluated with the PL spectrum shown in Figure 7b, which revealed that the VdWE-grown MoS 2 /WS 2 on the fused silica sample forms a type-II heterojunction (more detailed discussion is given in the Supporting Information).
It is very difficult to see the contrast between MoS 2 and WS 2 monolayers in the VdWE-grown MoS 2 /WS 2 heterostructures, since the VdWE provides uniform and continuous atomically thin TMDCs. To visualize the MoS 2 /WS 2 heterostructures, MoS 2 monolayer flakes were prepared on a 300 nm SiO 2 /Si substrate with the conventional CVD process, 38 followed by the coating with a uniform WS 2 monolayer with the VdWE process. The structure of these VdWE-grown WS 2 continuous film/CVD-grown MoS 2 flakes heterostructures illustrated in Figure S1(a) in the Supporting Information with the optical image in Figure S1(b) in the Supporting Information. The detailed characterizations of AFM, Raman, XPS, and PL are discussed in the Supporting Information ( Figure S1).
The spatial uniformity in the VdWE WS 2 /MoS 2 heterostructures are investigated by PL mapping. A recent report 39 has shown that the PL uniformity in exfoliated 2D materials is strongly correlated to the uniformity in the spectral properties, such as the emission energy and spectral weighting. A similar analysis is applied here to investigate the uniformity of the heterostructures, in terms of the emission energies of each of the corresponding layers in the heterostructure and offer a baseline for comparisons with future studies. Since there is an abundance of heterostructures flakes, the uniformity analysis extends naturally from intraflake (within one heterostructure flake) to interflake (between multiple flakes), which could provide additional insight for future growth optimizations. The monolayer MoS 2 flakes on this sample are mostly equilateral triangles, hexagrams, and partial hexagrams of various sizes and orientations. To sample this geometric distribution, an area is selected using optical microscopy, shown in Figure 8l that contains five numerically labeled flakes: flakes F1, F2, and F5 are triangles, F4 is a hexagram, F3 is a partial hexagram, and the regions outside of these flakes correspond to the VdWE WS 2 monolayer film. A PL map of the entire region was acquired, using a Horiba LabRAM spectrometer, with a 532 nm laser (637 kW/cm 2 , 5 s integration time), focused through a 100× 0.95 NA objective lens, and the emission dispersed with a 600 lines/mm grating. The mapped region is 40 μm × 40 μm in size, and the raster scan step size is 0.5 μm. Maps of individual heterostructure flakes were then isolated from the recorded PL map by a MATLAB program. Figure 8m shows that the WS 2 region has a single peak at ∼2.00 eV (WS 2 exciton), while two peaks appear in the heterostructure spectrum at ∼1.84 eV (MoS 2 exciton) and ∼1.98 eV (WS 2 exciton).
Spatial variations in emission energy are apparent for both MoS 2 and WS 2 , as revealed from the peak energy maps in Figures 8a−j. Across all flakes, the peak energy from both materials exhibits similar spatial patterns, where a local area that indicate blue-shifts (or red-shift) in one material corresponds to blue-shifts (or red-shift) in the other at the same spatial location. Although, for the MoS 2 peak, its intraflake energy range, taken as the 95% confidence region in the histograms shown in Figure 8k, is up to ∼10 meV, compared to ∼4 meV for that of WS 2 , from Figure 8n. There is a pronounced edge effect for WS 2 , less so for MoS 2 , where the peak appears to exhibit a significant blue-shift at the edge of all heterostructures measured. This also explains the differences that are apparent from the histograms plotted in Figures 8k  and 8n, showing largely monomodal distribution for MoS 2 and bimodal for WS 2 . The two modes in Figure 8n corresponds to the interior and edge peak energy distributions for WS 2 , and the means of these two modes are separated by ∼17 meV. The fact that all measured flakes exhibit similar behavior, independent of the flake size, geometry, and orientation, suggests that strain is the likely mechanism to explain this, as its magnitude could be changed at the edge WS 2 layer as its substrate changes from MoS 2 to silicon dioxide. For MoS 2 , the peak shift at the flake-edge is much less pronounced, up to 5 meV on average, which is smaller than the inhomogeneity in the MoS 2 peak energy of ∼10 meV, so that this modal separation is apparent only in the smallest flake measured (F5). Overall, the interflake uniformity is well-behaved, i.e., does not fluctuate significantly from flake to flake regardless of size geometry and orientation, which suggests that the growth process has good reproducibility between heterostructures. The intraflake uniformity is also well-behaved if the edge effects can be ignored, which could be valid for large-area heterostructure flakes. However, charge transport phenomena at the edge that change this behavior, which could be an interesting avenue to explore in a future study with a device, because PL-uniformity analysis alludes to the optical transport phenomena only.

CONCLUSION
In conclusion, we have demonstrated a scalable fabrication process for TMDC monolayers and their heterostructures by van der Waals epitaxy. These VdWE-grown MoS 2 , WS 2 monolayers, and their heterostructures have been successfully deposited on CMOS-compatible substrates, such as 300 nm SiO 2 /Si wafers, quartz glass, fused silica, and sapphire. Detailed characterizations of these TMDCs materials have been performed with SEM, AFM, XPS, micro-Raman, micro-PL, TEM, EDX, and SAED techniques and the band alignment and large-scale uniformity of MoS 2 /WS 2 heterostructures has also been evaluated with spatially resolved PL spectroscopy. These results have demonstrated not only the excellent characteristics of MoS 2 and WS 2 monolayers with large-scale uniformity but also the feasibility of large-scale TMDCs heterostructures that can be achieved by the VdWE in this work. We believe this process and resulting large-scale MoS 2 , WS 2 monolayers and their heterostructures have demonstrated promising solutions for the applications in next-generation nanoelectronics, nanophotonics, and quantum technology.